This disclosure relates generally to gas sensors, and, more particularly, to oxygen sensors for exhaust systems of mobile vehicles.
Gas sensors are used in the automotive industry to sense the composition of exhaust gases such as oxygen, hydrocarbons, and oxides of nitrogen, with oxygen sensors measuring the amounts of oxygen present in exhaust gases relative to a reference gas, such as air. A switch type oxygen sensor, generally, comprises an ionically conductive solid electrolyte material, a sensing electrode which is exposed to the exhaust gas, and a reference electrode which is exposed to the exhaust gas, and a reference electrode which is exposed to the reference gas. It operates in potentiometric mode, where oxygen partial pressure differences between the exhaust gas and reference gas on opposing faces of the electrochemical cell develop an electromotive force, which can be described by the Nernst equation:   E  =            (              RT                  4          ⁢                      xe2x80x83                    ⁢          F                    )        ⁢          ln      ⁡              (                                            (                              P                                  O                  2                                            )                        ref                                (                          P                                                O                  ⁢                                      xe2x80x83                                                  2                                      )                          )            
where:
E=electromotive force
R=universal gas constant
F=Faraday constant
T=absolute temperature of the gas
(Po2)ref=oxygen partial pressure of the reference gas
(Po2)=oxygen partial pressure of the exhaust gas
The large oxygen partial pressure difference between rich and lean exhaust gas conditions creates a step-like difference in cell output at the stoichiometric point; the switch-like behavior of the sensor enables engine combustion control based on stoichiometry. Stoichiometric exhaust gas, which contains unburned hydrocarbons, carbon monoxide, and oxides of nitrogen, can be converted very efficiently to water, carbon dioxide, and nitrogen by automotive three-way catalysts in automotive catalytic converters. In addition to their value for emissions control, the sensors also provide improved fuel economy and drivability.
The solid electrolyte commonly used in exhaust sensors is yttria-stabilized zirconia, which is an excellent oxygen ion conductor. The electrodes are typically platinum-based and porous in structure to enable oxygen ion exchange at electrode/electrolyte/gas interfaces. These platinum electrodes may be co-fired or applied to a fired (densified) electrolyte element in a secondary process, such as sputtering, plating, dip coating, etc. These electrodes can be made in the form of a film, paste, or ink and applied to the solid ceramic electrolyte in several ways. The electrode is added either before the ceramic is fired (green), before the ceramic is fully fired (bisque) or after the ceramic is fully fired. Once the electrode is added and fired, a strong bond should result between the electrode and the ceramic body. In the case of an oxygen sensor, poor bonding between the platinum and the yttrium stabilized zirconia body can result in poor adhesion leading to poor sensor performance and unacceptable durability.
The poor adhesion is due to the different coefficients of thermal expansion between the electrodes, the electrolyte, and the porous protective coating. For example, the platinum electrode has a different thermal expansion than the yttria-zirconia electrolyte. The varying degrees of thermal expansion results in a xe2x80x9cpullingxe2x80x9d phenomenon between the electrode and the electrolyte, increasing the debonding at the platinum and zirconia interface.
Furthermore, an electrical resistance between the electrode and the electrolyte exists in each of the electrochemical cells. Minimizing this resistance generally will result in an increase in the resultant electromotive force due to oxygen concentration variation between the exhaust gas and a reference gas. Reducing the applied voltage also has the effect of increasing the useful life of the sensors.
Additionally, impurities (also referred to as xe2x80x9cpoisonsxe2x80x9d) within the electrolyte flux to the outer surface of the electrolyte upon firing. This is particularly of concern where the electrode is added either before the ceramic is fully fired or after the ceramic is fully fired. The poison interferes with both the adhesion and electrical resistance of the sensor.
While existing sensors are suitable for their intended purposes, there still remains a need for improvements, particularly regarding the electrical and mechanical interface between the electrode and the electrolyte.
The drawbacks and disadvantages of the prior art are overcome by the exhaust gas sensor including a first electrode, a second electrode, and an electrolyte disposed between said first electrode and said second electrode, wherein the electrolyte includes a first portion disposed at least in partial physical contact and in ionic communication with a second portion, the first portion having a first portion grain size which is different than a second portion grain size.
A method for manufacturing a gas sensor includes forming a multiple portion electrolyte. The electrolyte is formed with a first portion having one grain size, and a second portion at least in partial physical contact and in ionic contact with the first portion, the second portion having a second portion grain size different from the first portion grain size. The electrolyte may be fired before or after application of an electrode in ionic contact with the first portion and a second electrode in ionic contact with said second portion